1. Field of the Invention
The present invention relates to an electromagnetic electroacoustic transducer and particularly to a configuration for attaining improvement in frequency characteristic of the electromagnetic electroacoustic transducer.
2. Background Art
Generally, an electromagnetic electroacoustic transducer includes a diaphragm made of a magnetic material, a magnet for generating a magnetostatic field to make the magnetostatic field act on the diaphragm, an electromagnetic coil for generating an oscillating magnetic field corresponding to an electric signal to make the oscillating magnetic field act on the diaphragm, and a casing for storing the diaphragm, the magnet and the electromagnetic coil therein. The electromagnetic electroacoustic transducer is formed so that an electric signal is converted into an acoustic signal by an electromagnetic transducer function.
In the electromagnetic electroacoustic transducer, a sound emitting hole through which a front space on a front surface of the diaphragm communicates with a front outer space in front of the casing is formed in the casing so that sound generated by vibration of the diaphragm is radiated to the front outer space in front of the casing by the sound emitting hole. On this occasion, if a rear space on a rear surface of the diaphragm is closed, sound pressure has a tendency toward decrease because an air damping effect prevents the diaphragm from vibrating sufficiently up to its vibration limit. Particularly when the size of the electromagnetic electroacoustic transducer is reduced, this tendency becomes strong.
Therefore, for example, as described in JPA9-149494, there has been heretofore proposed an idea that a second sound emitting hole through which a rear space on a rear surface of the diaphragm communicates with an outer space outside the casing is additionally formed in the casing to reduce air pressure of the rear space to thereby prevent reduction of sound pressure.
On this occasion, when the second sound emitting hole is formed so as to communicate with a front outer space in front of the casing, for example, as described in JPY1-16155, improvement in sound pressure can be attained by a resonance effect of the rear space on the rear surface of the diaphragm.
In JPY1-16155, no description is made on specific configuration for obtaining the resonance effect of the rear space on the rear surface of the diaphragm. On this occasion, a sufficient resonance effect cannot be obtained by only making the second sound emitting hole communicate with the front outer space in front of the casing, so that improvement in frequency characteristic of the electromagnetic electroacoustic transducer cannot be attained.